Study Bolsters Theory That Shellfish Remove Nitrogen from Ponds

As communities on the Island and across the region look to shellfish as a possible strategy to restore the health of coastal ponds, a new study on the Cape provides the numbers to back up their work.

Bivalves have long been known to attenuate nitrogen in their bodies by filter feeding. But the new study by scientists at the Woods Hole Sea Grant Program and Cape Cod Cooperative Extension, along with the Mashpee Department of Natural Resources, reveals just how much nitrogen they remove. It also highlights the effects of development along the coast.

Joshua Reitsma, marine program specialist at the Sea Grant and Cape Cod extension programs and the lead author for the report, emphasizes that nitrogen is an essential nutrient for shellfish and humans alike. “The problem is we are just getting too much of it, and that can have bad effects as we’re seeing in a lot of areas,” he told the Gazette this week.

Beginning in 2012, the study built in part on other studies that were able to identify the isotopic signature of nitrogen originating from wastewater, which is seen as the primary cause of declining water quality in coastal ponds in the region. Too much nitrogen leads to algal blooms, which prevent sunlight from reaching the bottom, and consume valuable oxygen when they decompose.

Previous studies had left town planners with a broad range of estimates. “They seemed to just be pulling numbers from all over the place,” Mr. Reitsma said. “We realized that there is a lack of local data.”

Published last week in the journal Marine Pollution Bulletin, the study provides the first comprehensive measurement of how much nitrogen oysters and quahaugs in the region remove from their environments. A local quahaug contains about 0.22 grams of nitrogen (an increase from previous estimates), according to the study, with local oysters containing about 0.28 grams each.

The study did not look at nitrogen in the animals’ feces, which enters the pond sediment and may be converted to nitrogen gas through decomposition. Mr. Reitsma said that process is highly variable, and hard to measure. “It’s definitely a benefit if it’s happening,” he said, but his groups lacked the necessary resources.

Oysters grown in the wild or on the bottom had more nitrogen than oysters grown in suspended cages, which Mr. Reitsma attributes to the need for thicker shells, which translates to more weight and more nitrogen. But most of the nitrogen in both oysters and quahaugs is contained in their soft tissue.

The numbers vary significantly by season, the study confirms, and to a lesser degree by location. Oysters harvested in the fall had about 98 per cent more soft tissue than those harvested in the spring, with quahaugs showing an increase of about 63 per cent. That translates to 44 per cent more nitrogen in oysters and 28 per cent more in quahaugs, as they fatten up for the winter.

Highlighting the effects of development, animals harvested on the south side of the Cape showed a significant increase in the amount of nitrogen from wastewater, compared to the north side, which has double or triple the tidal range and where ponds tend to be more open to the sea, helping to flush away excess nitrogen.

The study notes that the propagation and harvesting of shellfish in adequate numbers “may represent a method to help alleviate increasing nitrogen levels in local embayments.” It also notes the potential for more nitrogen removal in fall versus spring, and recommends that communities measure their progress by weight rather than total harvest, since nitrogen content varies with the size of an animal.

Mr. Reitsma said the data for the Cape provide a decent average for the Island. But he encouraged the communities here to measure their own harvests for accuracy. “It’s not going to be vastly different,” he said. “But it can make a difference in the end. When you are talking millions of shellfish it can add up.”

The town of Mashpee has already used the study to help develop a comprehensive watershed nitrogen management plan, which follows from the Massachusetts Estuaries Project, which has provided nitrogen reduction goals for most coastal ponds in the state. The plan includes the harvesting of millions of shellfish every year. Mashpee shellfish constable Richard York also contributed to the study.

“No one is really saying it’s going to be the sole source of cleanup,” Mr. Reitsma said. “But in some areas, the town of Mashpee really thinks this can be effective and they’re really going for it.” He added that the state Department of Environmental Protection appears to be on board with the use of aquaculture to meet the MEP goals, as long as there is a backup plan, such as expanded sewering or denitrifying septic systems to reduce the nitrogen load.

Falmouth, Barnstable, Yarmouth, Dennis, Orleans and Wellfleet on the Cape are also looking to aquaculture as a way to restore their coastal ponds.

On the Island, Edgartown and Oak Bluffs are using oysters to help restore Sengekontacket Pond, which has among the most impaired waters on the Island. Oak Bluffs launched its program with 250,000 oysters in 2015, and doubled that amount last year. Town shellfish constable David Grunden said another half million oysters are now overwintering in cages and will be distributed this spring or summer. Edgartown’s program started with 500,000 oysters in 2013.

To encourage participation, Oak Bluffs sets a separate half-bushel weekly limit for oysters, in addition to the regular half-bushel limit for recreational shellfishing.

“We’ve had people buying shellfish licenses just to get the oysters,” Mr. Grunden said, acknowledging the rougher appearance of the oysters, compared to the ones grown for market in Katama Bay and Edgartown Harbor. “As far as flavor and everything, it was nothing but positive responses.”

The Oak Bluffs oyster harvest runs from November to April, but Mr. Grunden said the focus so far has been on how much nitrogen each animal removes over the course of its life (about 1.5 or two years), rather than the time of harvest. He added that harvesting in the winter has the benefit of lowering the risk of the oyster disease vibrio, which thrives in warmer waters, and avoiding competition with the professional growers, whose products fetch a high price in the summer.

Mr. Grunden said the numbers in the study were at the lower end of what the town has been working with, but he agreed the study was local enough to provide a reliable benchmark going forward. Since around 2006, the overall water quality in Sengkontacket Pond has not improved, he said, but it also hasn’t gotten any worse.

Comments (1)

Joseph Paul Uranker, Oak Bluffs

The Gazette article by Alex Elvin Brotherhood of Oysters Is Delicious Thursday, June 30, 2016 - is a prime reason why the Brothers Dan and Greg Martino should be allowed in Lagoon Pond. Your earlier article by Olivia Hull September 16, 2014, Selectmen Allow Oyster Farm off Eastville discussed the blocking of location to quote the article "The Martinos first proposed a farm in Lagoon Pond last year, which they said would help reduce nitrogen in the pond, but stakeholders objected to that plan.
In spring of 2013, the shellfish committee said the proposed area was too large and would interfere with commercial and recreational shellfishing, as well as other recreational activity. The board also voted 4-1 not to allow any private aquaculture in the Lagoon, and suggested that the town pursue sewering around the pond to reduce nitrogen." Time now is the time to revise the allowing of them to relocate if they desire to do so.